1. Field of the Invention
The present invention biases to a lighting circuit for and a lighting method of a high-pressure discharge lamp whereby a both-end voltage (i.e., lamp voltage) of the high-pressure discharge lamp can be stabilized for a long period of time.
2. Background Art
A high-pressure discharge lamp is applied to, for instance, a projector. In principle, a high-pressure discharge lamp can be operated by means of both direct current and alternating current. However, an operation by means of alternating current is normally considered to be preferable to enhance efficiency of the high-pressure discharge lamp, because rapid loss in either of a pair of electrodes can be herein avoided in comparison with an operation by means of direct current.
However, increase and redaction in both-end voltage (i.e., lamp voltage) have been observed in a high-pressure discharge lamp even when the high-pressure discharge lamp was operated by means of alternating current. Such increase and reduction are phenomena that have been observed since the high-pressure discharge lamp was new. Especially, remarkable increase in the lamp voltage was observed when a projector was set in “a normal mode” for lighting the high-pressure discharge lamp at a rated power. This was attributed to the fact that both of the pair of electrodes disposed within the high-pressure discharge lamp were worn by applying the rated power to the high-pressure discharge lamp, and thereby, the distance between the pair of electrodes (hereinafter referred to as “inter-electrode distance”) was increased.
In terms of light utilization efficiency, a point light source is desired as a light source to be used for a projector. However, when the inter-electrode distance is increased in the high-pressure discharge lamp, a light emitting area is inevitably increased. Therefore, a drawback has been caused that light utilization efficiency is degraded and this results in great reduction in illuminance maintaining rate of the high-pressure discharge lamp in the beginning of usage. It should be noted that the illuminance maintaining rate hereinafter refers to a rate obtained by dividing “illuminance of light that is emitted from the high-pressure discharge lamp through an optical system and is effectively utilized in a given phase of the high-pressure discharge lamp” by “illuminance of light that is emitted from the high-pressure discharge lamp through the optical system and is effective utilized in a new product phase of the high-pressure discharge lamp”.
To avoid such a drawback, Japan Laid-open Patent Application Publication No. JP-A-2013-033605, for instance, describes a technology for modulating a lighting frequency of a high-pressure discharge lamp. In short, the technology for modulating a lighting frequency refers to a technology that is configured to light a high-pressure discharge lamp at a lower frequency when the lamp voltage of the high-pressure discharge lamp is increased (i.e., when the inter-electrode distance is increased), and by contrast, is configured to light the high-pressure discharge lamp at a higher frequency when the lamp voltage is reduced (i.e., when the inter-electrode distance is reduced).
According to the technology for modulating a lighting frequency (i.e., the technology described in the Publication No. JP-A-2013-033605), remarkable reduction in illuminance maintaining rate can be avoided in the beginning of usage by reducing variation in inter-electrode distance in the high-pressure discharge lamp.
However, when the technology for modulating a lighting frequency is now applied to a DLP (Digital Light Processing) method employed by most of the commercial projectors, chances are that an image to be projected is negatively affected. Specifically, when the technology for modulating a lighting frequency is actually applied to a DLP projector, a drawback is produced that appropriate projection is limited only at a frequency that synchronization with a color wheel is achieved. Suppose the technology for modulating a lighting frequency is applied to the DLP projector despite that synchronization with the color wheel is not achieved, an output of light to be entitled from the high-pressure discharge lamp varies (i.e., light gets dark) in reversal of the polarity of damp current (i.e., at zero ampere). Accordingly, flicker is supposed to be caused in a projected screen.
The present invention has been developed in view of the aforementioned drawback of the conventional art. Therefore, it is a main object of the present invention to provide a lighting circuit for and a lighting method of a high-pressure discharge lamp whereby remarkable reduction in illuminance maintaining rate can be avoided by reducing variation in inter-electrode distance in the beginning of usage without modulating a lighting frequency.